1. Field of the Invention
The present invention relates to a braking chopper for dissipating braking power away from the direct-current rails of an AC drive system, said braking chopper being an inverter unit physically similar to those used in normal motor supply applications.
2. Description of Background Art
Braking choppers are normally used in large AC drive systems where rectifier units (REC) connected to a supply network are connected via direct-current rails to inverter units (INU) controlling motors (FIG. 1). When the summed power of the INUs is negative, i.e. when the motors or at least some of them are braking, the braking power has to be dissipated away from the direct-current rails to avoid overvoltage. The commonest solution is to feed the braking power via a chopper into a braking resistor R (FIG. 2).
Prior-art braking chopper solutions are presented in FIG. 2:
1. A chopper unit BRC specifically designed for the purpose, with a power semiconductor switch V1 that starts to conduct when the voltage in the DC rails exceeds a set limit, and
2. A conventional INU with a special program loaded in it to accomplish a braking chopper function. In the example solution in FIG. 2, the INU is an inverter bridge in which the lower-branch power semiconductor switches V4 . . . V6 are connected in parallel to perform the same function as the switch in the BRC. Thus, no upper-branch switches are used in this solution at all. The use of a conventional INU as a braking chopper is a good solution e.g. in respect of maintenance and spare parts because it is a standard unit physically identical to those used as motor controllers.
The inverter units are designed and rated to serve as motor supply devices. In this application a considerable proportion, e.g. 40%, of the losses occurring in the power semiconductor switches consists of switching losses arising in switching situations. The amount of switching losses is influenced by the switching frequency and the inductivity of the load current, among other things. The load current of the motor is normally inductive, which adds to the switching losses.
In a braking chopper application, the load current is almost purely resistive and also the required switching frequency is considerably lower than in a motor drive. Therefore, the load of the power semiconductor switches in the prior-art solution is very low and thus the limit to the power to be fed into the braking resistor is set by the wiring and connections, which are designed in accordance with the needs of a motor application.